The maximum reach of long haul on-off keying transmission systems (such as a wavelength division multiplexed (WDM) transmission system operating at 20 Gbits/s or above) is typically limited by three types of signal distortion. Examples of such distortions are optical-to-signal noise ratio (OSNR) degradation, cross-talk and intersymbol interference (ISI). For the purposes of this discussion, ISI-related distortion are of further interest. ISI occurs when received data pulses are altered by pulse broadening or multi-path propagation (e.g., reflections) in the channel in which they travel. Pulse broadening can occur because of chromatic or polarization mode dispersions or receiver bandwidth limitations as described below. The results of such interference result in poor transmission characteristics.
One specific transmission characteristic that can drastically limit transmission in high-bit-rate systems, particularly for standard single-mode fibers (SSMF), is pulse-spreading type of ISI. This condition is shown in FIGS. 1a and 1b. Specifically, FIG. 1a depicts a graph 100 of the signal intensity I over time T for an input fiber of a WDM transmission system. The graph 100 is broken up into bit slots (denoted by sections 102, 104 and 106, respectively) which define the time slots available for data to be transmitted in such a system. At a first bit slot, bit slot 102, a “1” data bit (encoded in a non-return to zero (NRZ) format) is being transmitted followed by a “0” data bit in a second bit slot, bit slot 104. In a perfect WDM transmission system with perfectly formed fibers, the output intensity of these data pulses would be identical to the input data pulses shown in graph 100. Unfortunately, due to the above-identified interference conditions (pulse-spreading and the like), the signal sent to the output fiber is not identical to the signal sent to the input fiber.
More specifically, a graph of the signal intensity at the output fiber is seen in graph 120 of FIG. 1b. The graph 120 shows the same bit slots 102, 104 and 106 after the “1” and “0” data bits have passed through the transmission system. The resultant data stream is more rounded and not as well defined in the first bit slot 102 for the “1” data bit and furthermore extending into the second bit slot 104 which must represent the “0” data bit. ISI occurs when pulses propagating in the fiber spread out and overlap with each other. This condition can occur due to any of the above-described dispersion (or other) phenomena or by the bandwidth of a particular receiver being smaller than the system data rate (i.e., a 10 Gbits/s signal received by a 8 Gbit/s bandwidth limited receiver). The overlap, along with fiber nonlinearity, creates the ISI at locations of the “0” bits in a sequence of pulses, representing logical “1's” and “0's”. If the ISI becomes too large, it is detected by a receiver as logical “1's”, which can lead to transmission errors. These deleterious transmission characteristics are evidenced at the 10 Gbit/s transmission rate. As the bit rates of such a WDM transmission system increases, (i.e., for 20 Gbit/s, 40 Gbit/s and higher), the pulse broadening and subsequent ISI severely restricts the usable bandwidth of the system.
Methods of mitigating ISI and the effects it has on circuitry is found in various references including J. H. Winters, R. D. Gitlin, “Electrical Signal Processing Techniques in Long-haul Fiber-optic Systems”, IEEE Transactions on Communications, Vol. 38, Issue 9, September 1990, pp. 1439-1453; L. Möller, A. Thiede, S. Chandrasekhar, W. Benz, M. Lang, T. Jakobus, M. Schlechtweg, “ISI Mitigation Using Decision Feedback Loop Demonstrated with PMD Distorted 10 Gbit/s signals”, Electronics Letters, Nov. 25, 1999, Vol. 35, No. 24, pp. 2092-2093; H. Bülow, F. Buchali, W. Baumert, R. Ballentin, T. Wehren, “PMD Mitigation at 10 Gbit/s Using Linear and Nonlinear Integrated Electronic Equaliser Circuits”, Electronics Letters, Jan. 20, 2000, Vol. 36, No. 2, pp. 163-164; and J. H. Winters, S. Kasturia, “Adaptive Nonlinear Cancellation for High-speed Fiber-optic Systems”, IEEE Journal of Lightwave Technology, Vol. 10, No. 7, July 1992, pp. 971-977. However, as bit rates increase, such solutions will not be able to adequately compensate for the inevitable signal distortions.
As such, there remains a need for optical fiber transmission systems that can handle high-capacity communications while reducing the deleterious consequences of linear and/or non-linear distortions and PMD.